| MgB2, with a relatively high-superconducting transition temperature of 39K, has created the new superconducting transition temperature record in metallic compounds. Different from the cuprate-related high temperature superconductors. MgB2 has many advantages, for example, the relatively simple chemical composition and crystal structure, the longer coherent length, the small anisotropy. no weak-link behavior of grain connectivity, and lower cost of raw material and fabrication, and can carry high current. However, the critical current density Jc of MgB2 decreases rapidly with the temperature and magnetic field rising. It has been a hot concern in practical research of superconducting materials that how to introduce effective flux pinning centers in MgB2, thus improve the capacity of current carrier.To resolve the existing problems, introduction of effective flux pinning centers is needed. And carbon-doped materials can effectively improve the performance of MgB2 practical use. Therefore, this paper studied the effects of two kinds of carbon-doped materials (amorphous C and crystal C10H8) on the structure and superconductivity of MgB2 wires. And a systematic study on physical and chemical reaction mechanism of Mg-B system is carried on.MgB2-xCx (x=0,0.05,0.10,0.15) wires were fabricated by in-situ PIT method and sintered in Ar atmosphere with normal pressure. Phase structure, microstructure and chemical composition of the samples sintered at different temperatures were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The critical transition temperature (Tc) and hysteresis loop (M-H) were tested by Quantum Design's Physical Properties Measurement System.And the critical current density (Jc), the magnetic flux pinning force (Fp), the critical magnetic field (Hc2) and the irreversible magnetic field (Hirr) were worked out. The proposal that ideal reaction for MgB2 formation is solid-liquid reaction is put forward, for solid-liquid reaction is good for the reduction of pores, cracks and enhancement of grain connection. The best phase formation temperature range should be 650℃～750℃for in-situ PIT preparation of solid-state reaction to get MgB2. Samples sintered at this temperature range has tiny grains and good grain connection. The results show that carbon doping content x=0.1 (atomic ratio) can effectively improve the MgB2 performance.The effects of original powder (Mg powder and B powder) on the MgB2 microstructure and superconductivity were also studied in this thesis.Fabrication and phase formation dynamics of naphthalene C10Hg dopped MgB2 wires were systematically studied. Compared with carbon-doped MgB2, most naphthalene C10H8 vaporised out during the sample heat treatment for the melting and boiling points of naphthalene are 80.1℃.217.9℃respectively, far below the heat treatment temperature, therefore this dopping source can not break into the lattice. However, the naphthalene still can reduce the resistance of MgB2 above its critical transition temperature (Tc), and increase Mg-B phase formation temperature to 700℃. Sample of x=8 (quality ratio) dopping content heat treated at 750℃is superior to that of x=5 (quality ratio).At 15K,20K,25K, Jc of the doping content x=8 (quality ratio) sample achieves 1.5×10'A/cm2.1.1×105A/cm2 and 3.8×104A/cm2 respectively with zero field, and at the corresponding temperature, Jc of doping content x=5 (quality ratio) sample achieves 9.2×104A/cm2.3.1×104A/cm2 and 1.2×104A/cm2 under the zero field.While Jc of the doping content x=8 (quality ratio) sample declinds to 5.1×104A/cm2,1.8×104A/cm2 and 6.9×102A/cm2 respectively with 1.5×104Oe magnetic field at the same temperature, and under the corresponding conditions, Jc of doping content x=5 (quality ratio)sample shinks to 1.8×104A/cm2.5.9×103A/cm2 and 0A/cm2. In summary, Jc value of the two dopping content samples decline fast with outer magnetic field, indicating a lack of effective flux pinning centers. |
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